What can be most easily seen that change is constantly going on in a living language?

If you’re detecting change with your eyes: New vocabulary, then semantic shifts in existing words, then syntax — particularly syntax of individual words. fun became an adjective within my lifetime.

If you’re detecting change with your ears: all of the above, then maybe phonetics. But sound change is slower, socially and generationally stratified, and geographically uneven: most people don’t realise it even if they’re in the middle of it.

Pragmatics, phonology and morphology are much slower moving, though of course they change too.